How to read this ES6 code utsing destructuring and initialization?

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I ran into some code that looks like the following:

const {
  foo = []
} = this.options

Assuming in this case that this.options is an JavaScript Object, how does this work? Does all of this.options get assigned to foo and if this.options is undefined, does foo just get initialized to an empty array? I found this code confusing because this.options is not an Array but is instead an Object of key/val pairs.

Sometimes it helps to just try things out. What you'd observe is that a default value is assigned to foo in case it is missing within the to be assigned object

function one() {
    const options = {};
    const {
        foo = []
    } = options;

function two() {
    const options = {foo: 'bar'};
    const {
        foo = []
    } = options;

function three() {
    const options = {};
    const {
        foo = 'foo',
        bar = 'bar',
        baz = 'baz'
    } = options;
    console.log(foo, bar, baz);


Destructuring assignment, User must explicitly enable this feature. See also. Assignment operators · "ES6 in Depth: Destructuring"  The following code snippet shows how we can use nested object destructuring to do this. Here, we defined three local variables: name , maths and science . Also, we specified a default value of 50 for science in case it does not exist in the nested scores object.

From MDN (emphesis mine ) :

The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

Not all of this.options get assigned to foo, it's unpacking foo from the Object :

const options = {
  foo: ['foo', 'bar'],
  bar: 'hello world'

const {
  foo = []
} = options;


ES6 In Depth: Destructuring - Mozilla Hacks, With destructuring assignment, the equivalent code becomes more of is when you are using destructuring on an object to assign variables, but So, as you can see, destructuring is useful in many individually small cases. Destructuring. Destructuring is an excellent way to extract data stored in object s and arrays. Is very useful to read a complete structure in only one line. Destructuring assignment. Is supported in node since ~v6.4.0, Chrome #49 and Firefox #41. Let's start! Array destructuring

If you run it through babel you'll see that if exists then it will be bound to the name 'foo' in that scope and if it doesn't then foo is set to an empty array.

ES6 Destructuring: The Complete Guide, Apr 18, 2018 · 9 min read You may opt to test the code snippets on Codepen or your favorite online JavaScript live editor. Another alternative would be using the Using destructuring, the function from our previous snippet will look like this:  Craig's latest tutorial explains how to use complex-sounding ES6 destructuring assignment shorthand syntax to simplify your code and save typing effort.

Here is an in-depths article for you to better understand ES6 Destructuring

Destructuring assignment allows you to assign the properties of an array or object to variables using syntax that looks similar to array or object literals. This syntax can be extremely terse, while still exhibiting more clarity than the traditional property access.

For your sample script the assignment are looking for an object with property "foo" from right side. If it can not find it, it will assign foo with default value as empty array.

If the left side is null or undefined, the operator will throw an error "Uncaught TypeError: Cannot destructure propertyfooof 'undefined' or 'null'."

Destructuring and parameter handling in ECMAScript 6, ECMAScript 6 (ES6) supports destructuring, a convenient way to extract Note that we are using the same syntax that we have used for constructing. Don't forget that the iterator over strings returns code points (“Unicode You can more easily see why things work this way if you replace the pattern  Beginner's Guide to ES6 Destructuring ES6 comes with a bunch of new features, one of which is destructuring. This is a very handy way to extract items from objects and arrays which -once understood- can make your code much cleaner and more readable.

// A regular JS object.
this.options = {
    'a': 'a',
    'foo': 'bar',
    'z': 'z'
console.log(' =',;
// === 'bar'

// Get 'foo' out of this.options. Becomes undefined if 'foo' doesn't exist in this.options.
const { foo } = this.options;
console.log('foo =', foo);
// foo === === 'bar';
const { nope } = this.options;
console.log('nope =', nope);
// typeof nope === 'undefined'

// Get 'zzz' if it exists, otherwise fall back to 1234.
const { zzz = 1234 } = this.options;
console.log('zzz =', zzz);
// zzz === 1234;
// zzz was set by its default fallback value.

Destructuring assignment, Destructuring assignment is a special syntax that allows us to In the code above, the second element of the array is skipped, the third we can add one more parameter that gets “the rest” using three dots "" : this happening or was this possible in es6 not now in latter versions? read sort method in js. ES6 Javascript offers a battery of exciting features that allow us to write code in clean, dry patterns with maximal reusability. One such way to use ES6 to write clean code is through the use of…

10. Destructuring, of an assignment). How to extract the values is specified via patterns (read on for examples). This is unfortunate when using object destructuring in an assignment: Why not as follows – which is also completely legal ES6 code? function  I just want to demonstrate that after firstname and lastname there are more initialization if that makes sense. – Lim H. Sep 5 '15 at 18:13 this cannot be assigned to - in ES5 never, in ES6 the only thing that changes its "value" is super() .

ES6 in Action: Enhanced Object Literals, The ability to create JavaScript objects using literal notation is Object Initialization From Variables ES6 code const a = 1, b = 2, c = 3; obj = { a b c }; // obj.a = 1, obj.b = 2, ES6 supports destructuring: you can create a variable with the same Whether you find this code easier to read is another matter! ES6 In Depth is a series on new features being added to the JavaScript programming language in the 6th Edition of the ECMAScript standard, ES6 for short.. Editor’s note: An earlier version of today’s post, by Firefox Developer Tools engineer Nick Fitzgerald, originally appeared on Nick’s blog as Destructuring Assignment in ES6.

ES6 tips and tricks to make your code cleaner, shorter, and easier to , This makes life far simpler and code easier to read. You can Missing or undefined values are initialized with the default value. Just be This is done using a 'key: variable' pairing on the left of the equals sign. function ES6 destructuring and concise parameters let { name, age, job: {company}} = foo();. Browse other questions tagged javascript ecmascript-6 destructuring es6-modules or ask your own question. The Overflow Blog Podcast 246: Chatting with Robin Ginn, Executive Director of the OpenJS…